About Those Vaccine Strains in Measles Outbreaks…

For one thing, you have to confirm that everyone with measles symptoms actually has measles. If you miss anyone, because measles is so contagious, then they could expose other people and the outbreak will keep getting bigger.

How do they confirm who has measles and who doesn’t?

While you could just make a clinical diagnosis, simply relying on the person’s history and pattern of symptoms, typically everyone just gets tested.

About Those Vaccine Strains in Measles Outbreaks…

What kind of testing?

“Laboratory confirmation is essential for all sporadic measles cases and all outbreaks. Detection of measles-specific IgM antibody and measles RNA by real-time polymerase chain reaction (RT-PCR) are the most common methods for confirming measles infection. Healthcare providers should obtain both a serum sample and a throat swab (or nasopharyngeal swab) from patients suspected to have measles at first contact with them. Urine samples may also contain virus, and when feasible to do so, collecting both respiratory and urine samples can increase the likelihood of detecting measles virus.”

It depends, but often a throat swab or a throat swab and urine are collected for PCR testing, especially if it has been 7 or fewer days since the patient came down with their rash.

If it has been longer than 7 days, then testing using urine and blood specimens can be performed.

“Molecular analysis can also be conducted to determine the genotype of the measles virus. Genotyping is used to map the transmission pathways of measles viruses. The genetic data can help to link or unlink cases and can suggest a source for imported cases. Genotyping is the only way to distinguish between wild-type measles virus infection and a rash caused by a recent measles vaccination.”

Measles For Healthcare Professionals

Because many people get vaccinated during an outbreak and a rash and fever is a side effect of the MMR vaccine, testing becomes very important in those who were recently vaccinated.

Statistics from the 2015 Disneyland measles outbreak. Anti-vaccine folks, this slide doesn’t mean what you think it means.

After getting vaccinated, testing helps confirm that someone has vaccine strain measles and not wild type, a vaccine reaction, and don’t actually have measles.

“Here, we describe a real-time reverse transcription-PCR (RT-PCR) method that detects the vaccine genotype (MeVA RT-quantitative PCR [RT-qPCR]) and that can provide rapid discrimination between wild-type-virus infections and vaccine reactions.”

“During outbreaks, measles vaccine is administered to help control the outbreak, and in these situations, vaccine reactions may be mistakenly classified as measles cases.”

CDC on Genetic Analysis of Measles Viruses

So the reports that you might have seen that 31 people in the California measles outbreak had a vaccine strain of measles aren’t true. There were at least 31 people who were recently vaccinated during the outbreak and had a rash and/or fever, and they tested positive for the vaccine strain, proving that they weren’t actually part of the outbreak. None of them actually had measles though.